The invention relates to a device for determining exposure values in an X-ray panoramic and/or cephalomretric apparatus for imaging patient""s head or a part thereof, which apparatus comprises an X-ray tube, an exposure control means providing a tube voltage, a tube current and an exposure time, and adjustable support means for a patient""s head, said support means being provided with at least two support elements with a variable spacing therebetween on the opposite sides of the patient""s head, and said support means being adapted for positioning against the patient""s head prior to an exposure. The invention also relates to a method for determining exposure values for an X-ray panoramic and/or cephalometric imaging of a patient""s head, in which method: an individual spacing between the opposite sides of the patient""s head is detected; exposure values are defined dependent on said detected spacing; and said exposure values are used for the imaging of the patient""s head or a section thereof.
When an X-ray image is exposed the tube current (mAxe2x80x94value) and tube voltage (kV xe2x80x94value) of the X-ray tube and the exposure time (seconds) of the image receptor, called in general form as exposure values, shall be in a proper level in order that the exposed image would have a good quality. The exposure values depend on e.g. the bone thickness of the individual person or patient, who is in each case the target of the imaging. The most simple method to determine the exposure values, i.e. the tube current and the tube voltage and the exposure time, is to manually evaluate the size the person/patient in question, and thereafter to adjust the exposure values accordingly, whereupon for a taller person, like adult men, a higher tube current and/or tube voltage and/or a longer exposure time is selected, for a person of a medium size, like adult women, a somewhat lower tube current and/or tube voltage and/or a shorter exposure time is selected, and for children considerably lower tube current and/or tube voltage and/or a shorter exposure time is selected. This selection according to the type of the person can be marked on a tube current and/or a tube voltage selector and/or on an exposure time selector effecting on a timer or rotational speed control unit of the panoramic/cephalometric apparatus, whereupon the exposure value selection is a very easy and rapid procedure. The accuracy of this kind of manual exposure value selection is however very low, because of the personal evaluations by individual operators, which is a serious drawback.
Accordingly many different exposure automatics have been developed, as disclosed e.g. in documents U.S. Pat. No. 5,386,448, U.S. Pat. No. 5,828,720 and EP-0 229 972 B1, which automatics have that common feature, that the radiation penetrated through some portion of the patient, either within the target area or in an area nearby the target area of the patient, is measured with a radiation detector and the detected radiation intensity is used for adjusting the exposure values. The accuracy of these methods are generally very good, because they measure the actual radiation which is to be received by the radiation sensitive film or image plate. These types of exposure automatics have complicated measuring circuits and expensive components, and further the operators shall have good competence and carefulness in use and maintenance of the apparatuses.
An alternative apparatus and method is suggested in document DE-30 35 435 A1, which describes a measuring gauge comprising a fixed scale, which has a measuring jaw in one end thereof, and a movable scale, which has another measuring jaw parallel with the jaw of the fixed scale, whereupon one of the scales is provided with a measure graduation including the values of the tube voltage and/or tube current needed for an X-ray panoramic imaging, and the other of the scales is provided with a reading element for reading the measure graduation, and whereupon the gauge has such dimensions that the jaws can be positioned against the opposite sides of a patient. This gauge is so in principle just like those gauges, which are in general use e.g. in the engineering industry, with the exception that it is graduated using voltage/current values instead of mechanical dimensions. The accuracy as such can be acceptable, but the use of the gauge is time consuming and transferring the exposure data from the gauge to the X-ray imaging apparatus can lead to human errors if not performed carefully enough.
So the first object of the invention is to minimize the possibilities to human errors and to provide an acceptable accuracy for exposure values of the X-ray tube as compared to the size of the patient in a panoramic and/or cephalometric imaging, in which the target of the image is within the area of patient""s head. In this context the jaws with teeth and/or the neck area, like cervical spinea with the lower cranial section, and/or the whole head of a patient are typically the targets, from which images shall be taken. The second object of the invention is to minimize the time used for the measuring, which time should approach nil, if only possible. The third object of the invention is to enable such measuring conditions that the size difference among adults and children respectively would not effect erroneous exposure values, and that special calibrations would not be necessary for various market areas. It is further an object of the invention that the measuring device with the data transfer could be achieved by simple and reliable arrangements.
The above-described problems are eliminated and the above-defined objects are achieved by means of a device of the invention, which comprises: a sensor connected to said two support elements and providing an electrical output proportional to the spacing of said support elements and accordingly proportional to dimension of the patient""s head therebetween; and a control unit or a connection feeding said electrical output transformed or non-transformed to the exposure control means, so adjusting the current and/or the voltage of the X-ray tube and/or the exposure time to be at one of a plurality of predetermined levels, each of which being in a predetermined relationship with the spacing of said support elements. The above-described problems are eliminated and the above-defined objects are achieved by means of a method of the invention, which prior to exposure comprises the steps: while resting the patient""s head against support means in an X-ray apparatus, at least support elements of a support element pair of said support means, said support elements being connected to a sensor, are moved towards each other and against the substantially opposite sides of the patient""s head; an electrical output from the sensor, representing the spacing of the support elements and accordingly the spacing between the opposite sides of a patient""s head, is fed to adjust or display a current and/or a voltage of an X-ray tube and/or to adjust or display an exposure time of the radiation from the X-ray tube.
According to the invention the problems described are surprisingly solved by detecting one outer dimension, typically the width of the patient""s head utilizing the adjustable head support elements inherent in every the X-ray panoramic and cephalometric apparatuses, transforming this mechanical dimension data by simple electrical unit or sensor into electrical data, which is proportional to the mechanical dimension, and transferring this electrical data in such configuration into the exposure control means of the apparatus that the tube current and/or tube voltage and/or the exposure time is altered or adjusted to a level, which is correct for image receptor. This device and method of the invention so creates exposure automatics, which is indeed very simple and requires a minimum of additional components as compared to standard panoramic and cephalometric apparatuses. The accuracy of this invented device and method is good enough in practice. The device and method of the invention further eliminates the effects of human inconsistencies concerning the operator, because it is an automatic system in question.